The present invention is related generally to systems and methods for communicating with nodes in a linear network linked by a fluid carrying channel or pipe, and in particular to such a system and method for communicating among and controlling a plurality of railroad cars in a train, where the railroad cars comprises a linear network.
For many years, railroad freight trains have operated with pneumatic brakes for braking the locomotive and the individual railcars. In a typical system, the locomotive supplies pressurized air to the railcars (specifically, to a railcar reservoir within each railcar) through a brake pipe extending the length of the train. The brake pipe of each railcar is serially connected to the brake pipe of the adjacent railcars via a flexible hose connector, sometimes referred to as a glad hand. At each railcar, a control valve responds to the brake pipe pressure for applying or releasing brake shoes against the individual railcar wheels. The brake pipe therefore serves to both supply the pressurized air to each railcar for driving the brake cylinders, thereby applying the brake shoes against the railcar wheels, and also as the medium for communicating brake application and release instructions to each railcar. In a typical prior art pneumatic brake system, the locomotive operator commands the railcars to apply their air brakes by creating a pressure drop of approximately seven to nine pounds in the brake pipe. Each railcar senses the drop in air pressure as it propagates along the brake pipe and supplies pressurized air from the local railcar reservoir to the wheel brake cylinders. The brake pressure applied to the railcar wheels is proportional to the change in the brake pipe pressure in the brake pipe. To release the brakes, the operator increases the pressure in the brake pipe, which is interpreted by the individual railcars as a command to release the brakes.
FIG. 1 illustrates a typical prior art brake system employed by a railroad freight train. A train brake system 202 comprises a locomotive brake system located on a locomotive 100 and a car brake system located on one or more railcars, illustrated by a car 200. The application and release of braking action is controlled by an operator within the locomotive 100. The locomotive 100 contains an air brake control system 102, including a controllably pressurized brake pipe 101 extending the length of the train, through which pressurized air and brake instructions are supplied to each of the cars 200. The brake control system 102 also includes an air supply input 111 for supplying, fluid (air) under pressure to charge the brake pipe 101. Ultimately, as will be explained further below, the air brake control system 102 controls the operation of the pneumatic-operated brake shoes 233 at each of the wheels 235 of the car 200.
Outside air is supplied via an air supply link 111 to an input port 121 of a relay valve 117. A bi-directional port 122 of the relay valve 121 is coupled to the brake pipe 101 via a link 109. The relay valve 117 further includes a port 123 coupled through an air pressure control link 103 to an equalizing reservoir 105. The pressure control link 103 is also connected to a pressure control valve 107 through which the equalizing reservoir 105 is charged and discharged during a brake operation. A exhaust port 124 of the relay valve 117 is controllably vented to the atmosphere. Coupled to the brake pipe 101 are various pressure measuring and display devices not germane to the present invention, and therefore not shown in FIG. 1.
The components of the railcar air brake control system 202 include a control valve 203 having a port 221 coupled to the brake pipe 101. The control valve 203 also includes the port 222 coupled to a pressure storage and reference reservoir 205. Finally, the control valve 203 includes a port 223 coupled to the air brake cylinder 231 for controlling the movement of the brake shoe 233 against the wheels 235 of the car 200. The brake system is initially pressurized by operation of the pressure control valve 107, which controls the air supply to the control link 103 so as to fully charge the equalizing reservoir 105. The relay valve 117 is then operated to couple the port 121 with the port 122 so that air is supplied therethrough to the brake pipe 101 charging the brake pipe fluid path to the predetermined charging pressure. This pressure (typically 72 psi) is established by the pressure of the equalizing reservoir 105 in the locomotive 100. Specifically, when the pressure at the port 122 matches the pressure at the port 123 the brake pipe is fully charged. Through the operation of the control valves 203 in each car 200, the pressure storage and reference reservoir 205 in each car 200 is fully charged to establish a reference pressure at the railcar 200.
When the locomotive operator desires to apply brakes to the wheels 235 of the railcars 200, he operates the pressure control valve 107, typically via a hand-operated control valve lever, causing a partial venting of the air pressure control link 103 and thereby a reduction in the pressure within the equalizing reservoir 105. This reduced pressure in the equalizing reservoir 105 is sensed by the relay valve 117 at the port 123. In turn, this causes the bi-directional port 122 to be coupled to the exhaust port 124, exhausting the brake pipe 101 to the atmosphere until the pressure within the brake pipe 101 equals the pressure of the equalizing reservoir 105.
As the pressure in the brake pipe 101 drops, the control valves 203 in each of the cars 200 senses the pressure reduction by comparison to the pressure in the pressure storage and reference reservoir 205. Since the pressure in the brake pipe 101 is less than the pressure in the pressure storage and reference reservoir 205, there is a corresponding increase in the pressure applied to the brake cylinders 231 from the port 233, resulting in an application of the brake shoes 233 against the wheels 235 in proportion to the sensed pressure reduction. Further pressure reductions in the equalizing reservoir 105, under control of the operator, produce corresponding pressure reductions in the brake pipe 101, and thereby the application of additional braking effort by the brake shoes 233 in each of the cars 200. In summary, the intended operation of the brake system in the cars 200, and specifically the braking effort applied at each of the cars 200, is proportional to the reduction of pressure in the equalizing reservoir 105.
To release the train car brakes, the operator operates the pressure control unit 107 to effectuate a recharging of the air brake control system 102. This is accomplished by bringing the pressure within the equalizing reservoir 105 back to its fully charged state as described above. With the equalizing reservoir 105 recharged, there is again a pressure differential (but opposite in sign to the previous pressure drop) between the ports 122 and 123 of the relay valve 117. This increase in pressure is sensed by the control valves 203 in each of the railcars 200, and in response the brake shoes 233 are released by the action of the brake cylinders 231.
The foregoing described pneumatic braking system has been used for many years and has the advantage of being entirely pneumatic. Such systems however are known to have certain disadvantages. For example, because the brake command signal (either an increase or decrease in air pressure) is a pneumatic signal, it must be propagated along the brake pipe. Accordingly, on long trains it can take many seconds for the brake application or release signal to propagate to the end of the train. Thus, during the propagation interval not all the cars in the train are braking or releasing. Generally, the braking signal propagates much slower than the speed of sound and therefore may require over one minute to propagate along a train of 150 cars. Because the applied braking force is a function of the pressure change detected at each railcar, the precision to which the brake application can be controlled is degraded both by the propagation characteristics of the brake pipe and leakage in the closed pneumatic brake pipe system. Further, in a typical prior art pneumatic braking system, there is no provision for partially releasing the brakes. Once the brake release signal is received via the brake pipe, each railcar fully releases its brakes. In some situations, it would be desirable for the train operator to affect only a partial release, such as when excessive braking has been applied, but it is desired to reduce the level of braking without fully releasing the brakes. The ability to partially release the brakes provides the train operator with improved and more precise control over train operation. Also, most prior art railway braking systems do not provide braking pressure variability among the railcars. Generally, all railcars apply the same braking force based on the sensed brake pipe pressure. But, some railcars will decelerate faster than others, e.g., empty cars decelerate faster than loaded railcars. The differential railcar deceleration rates generate considerable forces (called “slack action”) between the railcars and imposes extraordinary stresses on the car draft gear and coupler. The intra-train forces generated by these variable effects require that train operators brake the train judiciously, at a deceleration rate less than what might otherwise be desirable, solely to avoid these forces and the possibility of uncouplings, broken couplers (resulting in increased maintenance) and derailments.
Certain railcars include a retainer valve for imposing a partial brake release condition. The valve, when manually activated, retains some brake pressure in the brake cylinder even though the main brake valve is in a released state.
During the last several years, electronic-based improvements have been introduced to railway power and braking systems. For example, a system is available to provide communications between multiple locomotives located remote from each other in the train consist, so that a single train operator controls the throttle and locomotive brakes of the head-end and the remote locomotives. The system utilizes a radio frequency (RF) link between the lead locomotive (also referred to as the head end unit) and the remote locomotives to provide throttle and brake control. This system provides more even pulling of the railcars and improved locomotive braking performance, because each locomotive generates a pneumatic brake instruction in response to the received RF communication signal (which travels at the speed of light) from the lead locomotive, rather than from the slower brake signal conveyed along the pneumatic brake pipe. Since the brake instructions are generated nearly simultaneously at each locomotive on the train, the railcars receive the brake instruction earlier, as compared to a completely pneumatic system, relying solely on the brake pipe for propagation of the braking signal.
In recent years, the American Association of Railroads (AAR) and certain individual railroads have investigated the use of electronically controlled pneumatic (ECP) brake systems. Such systems typically provide brake commands via the propagation of an electrical signal over a wire extending the length of the train or via a radio frequency system operative between RF transceivers on the locomotive and on each railcar. The primary benefit of these ECP brake systems is the ability to activate the brakes on each car of the train using a signal that propagates at the speed of light. Thus the ECP brake systems allow for the nearly instantaneous activation of railcar brakes along the entire train.
Although wire-based ECP systems provide the benefit of braking signal propagation at the speed of light, the wires that carry the braking signals from car to car are subject to harsh environments and are therefore susceptible to damage. Each railcar glad hand includes an electrical connector for mating with the connector of the next railcar in the train consist to provide a continuous electrical path along the train. If a break or discontinuity develops in the wire, an emergency brake application is automatically initiated and train movement is halted until the break is found and repaired.
In lieu of a wire-based communication system, certain ECP braking systems send brake application and release commands via a radio frequency link to each railcar, where each railcar includes a transceiver for receiving the RF signal, forming a node in a linear token-based communications network. In one embodiment, synchronous communications in the form of a multi-hop network is employed to send a token outbound and inbound on the train consist. This network, once established, provides commands and receives status information from all the nodes (i.e., locomotives and railcars) in the network. Any remaining time/bandwidth network resources, when not in use by the ECP braking system, is available for other network clients such as providing distributed power commands or reports from railcar sensors. The hopping methodology transmits the command repeatedly to the nearest neighbor railcar as it leapfrogs toward its destination, that is, the end of the train or the front of the train. In practice, the network has been demonstrated to be very robust in its application to provide wireless command, control and status for train braking and other train systems.
However, train operation occasionally places certain nodes out of communication with other nodes. For example, as the train passes through a tunnel, certain nodes will be unable to communicate with others. Man-made obstructions in urban environments occasionally block the line-of-sight required for radio frequency communications between the head-end unit (i.e., locomotive) and distant nodes (i.e., railcars). Natural objects, such as mountains, may also be interposed in the line of sight. Thus it is not always possible to guarantee continuous radio frequency communications between all the cars in a railroad train.
Notwithstanding the difficult RF environment in which a train operates, certain classes of network signal protocols may require a high degree of network reliability. Messages may need to be received by all the nodes to affect certain actions at each node. For example, if a radio frequency communication system transmits braking commands from a locomotive to other locomotives or railcars within a train, it is critical that the commands be rapidly and reliably communicated to the destination nodes. In one protocol, receipt of such messages is confirmed by transmitting an acknowledgement signal from the receiving node back to the sending node. However, in the linear network topology of a train, where certain nodes may have an inadequate radiated power, and where the RF environment is constantly in flux, the receiving node may not be able to successfully send an acknowledgement message back to the sending node. Upon failing to receive an acknowledgment signal, the sending node resends the command, resulting in the unnecessary reuse of the available messaging bandwidth.
Also, mobile linear networks, such as a railroad train, present problems not encountered with a fixed network topology. For example, if a train encounters other trains as it moves along a track, the radio frequency signals from the two trains may interfere, preventing signals from the sending unit of the first train from reaching the railcars of the first train. Also, in those situations where similar or identical communications systems are used on each train, RF signals transmitted by the sending node of the first train and received by a railcar of the second train can cause unintended operations at the second railcar, for example, a brake application. Thus, the dynamic nature of this nodal RF communications environment presents certain disadvantages to reliable RF communications among the nodes of the train.
Certain train operations require that the train railcars be linked or ordered. To “link” the train, the head end unit (typically the locomotive) contacts each railcar and receives a response signal, which serves to order the cars in the train consist. Another objective of the linking process is to test the continuity of the brake pipe.
The simplest prior art technique for accomplishing train linking is the manual inspection and recording, in order, of each railcar number in the train. With trains extending over a mile and a half in length, this can be a burdensome and time-consuming task. Also, a manual list is prone to errors and missing a car in the train consist or incorrectly ordering the cars is more likely to occur during the ordering process for a long train. Another linking technique is initiated by sending a radio frequency signal from the head end unit, where the signal instructs each of the railcars to watch for a brake pipe pressure change, A pressure actuator in the locomotive creates the brake pipe pressure change and it propagates down the railcars. As each railcar detects the pressure change, it responds with a radio frequency signal back to the head end unit. Since the brake pipe pressure change propagates serially along the brake pipe, the RF responses are ordered accordingly and the train is linked. The RF return signal provides information about the railcar, the railcar number, for example, for use in ordering the train cars.
The linking process is especially important for ECP brake systems, (whether the brake signal is carried over a wire or via an RF link). Each railcar has a unique identity and is therefore individually addressable via the ECP communications system. The head end unit (HEU) or master node prompts each individual addressable car for operational and status information, but to take full advantage of this data it is necessary to know the location of the railcar in the train. A railcar manifest list can be used to create a file in the locomotive controller in which each railcar is identified (by railcar number, for example) and ordered.